www-client/firefox: use wrapper

www-client/firefox: use wrapper

This change will replace /usr/bin/firefox symlink with a
wrapper script:

- Using a wrapper will allow us to set additional variables
  more easily.

- The wrapper will address an issue that prevented external applications
  from opening links when a Firefox instance was already running.

- In general, the wrapper will allow user to execute 'firefox' from
  wherever and whenever they like without getting an error that an
  instance is already running.

Signed-off-by: Thomas Deutschmann <whissi@gentoo.org>